CHAPTER XI 

 LAKE TO FOREST OR PRAIRIE 



S WAS noted in discussing the inter- 

 dunal ponds the ultimate fate of pond 

 or lake is to be filled with the outwash 

 of soil from the surrounding hills and 

 the accumulations of vegetable debris. 

 Such a filling lake may give rise either 

 to (i) a forest, as shrubs and trees es- 

 tablish themselves in the firm soils of 

 its rising margin and push their way farther and farther out as 

 filling proceeds, or (2) to prairie. Indeed, these two associations, 

 the wet forest a/id the low prairie, may develop at different 

 parts of the same filling lake as is seen, for instance, at Wolf 

 Lake near Chicago. 



Beginning with the open water and running shoreward, the 

 zones in the filling pond that lead to the forests might be named 

 as follows: (i) open water area, or from the animals predomi- 

 nating, the Pleurocera area; (2) zone of submerged plants or 

 the aquatic insect zone; (3) water lily zone or painted turtle 

 zone; (4) the rush zone the marsh wren zone; (5) the cat- tail 

 zone red-winged blackbird zone; (6) the shrub zone, the katy- 

 did zone; (7) the ash-elm zone, the green heron zone. 



Out in the open water where the bottom is sandy, the water 

 shallow and largely free from invading plants, caddis-fly nymphs 

 are abundant, crawling over the bottom, as are also such snails 

 as Pleurocera and Goniobasis, while Vivipara contectoides is occa- 

 sionally found (Fig. 324). All three of these are operculate. In 

 Pleurocera the aperture is produced into a canal. P. elevatum 

 has nine to ten whorls, P. subulare twelve. Goniobasis livescens 

 is broadly conical and the aperture is rounded in front. Vivipara 



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